Experimental Evidence of the Effect of Surface Waves on the Airflow

Abstract

Experimental evidence of the effect of surface waves on the airflow is presented. According to the quasi-linear theory of wind-wave generation, the stress of airflow over surface gravity waves depends on both wind speed and the wave-induced stress. Here the wave-induced stress is related to the rate of change of wave momentum due to the wind, and for young wind sea, the wave-induced stress may be a substantial amount of the total stress in the surface layer resulting in a considerable enhancement of the drag of airflow. During HEXMAX, wind speed U10, friction velocity u*=τ½, and the one-dimensional frequency spectrum were measured simultaneously. The directional distribution was assumed to be given by the expression proposed by Donelan et al. (1985). Using the measured wind speed and frequency spectrum, it was possible to determine from quasi-linear theory the surface stress and to compare it with the observed stress. Very good agreement between observed and modeled stress is found, much better than when the Wu (1982) formula for the drag coefficient is used.